High-harmonic fingerprints of sharp spin twists in a chiral soliton lattice

Abstract

A magnetic field applied perpendicular to the helical axis of a monoaxial chiral helimagnet compresses the spin helix into a coplanar chiral soliton lattice (CSL). We show that optically driven high-harmonic generation from itinerant electrons coupled to a frozen CSL resolves the lattice-scale structure of the localized twist, rather than the continuum soliton shape. High-order harmonics remain perturbative for the uniform helix. As the winding localizes at fixed magnetic period, they grow by many orders of magnitude and acquire a nonperturbative dependence on the drive amplitude. The growth originates from a spatially nonuniform effective hopping that turns each soliton into a localized dip in the hopping amplitude. When the degree of winding localization is held fixed, high-order intensities fall by many orders of magnitude as the magnetic period increases toward the continuum limit, where the hopping modulation is spatially smoothed. High-order harmonics thus resolve a real-space characteristic of the coplanar CSL, even in the absence of scalar chirality and an emergent magnetic field.